in one of the last EDD versions a seller notifications were added. The text of this notification can be altered by the "edd_admin_purchase_notification" filter, but there's no filters to modify the headers or subject of such emails.
One of the results is that seller notifications is always a text/plain and can't include HTML tags (the buyer's notifications are HTML-allowed).
Right now we're fixing this by some additions in the
edd_email_purchase_receipt($payment_id, $admin_notice = true)
but it's clearly wrong. Can you add some more filters in this function?